In order to obtain paper having characteristic good printability, it is necessary that the surface of the paper be smooth, and substantially free of indentations or valleys. Smooth papers are required for clear printed image formation and also for good transfer of ink to the paper. Methods are well-known for coating paper with pigment slurries, wherein said pigment slurries fill in the valleys of the rough paper. These pigment slurries in addition to rendering the paper smooth, are designed to improve the whiteness of the paper, its printability, its pick strength, its water resistance, and its ink holdout. Further, it is essential that the coating material is receptive to ink, i.e., it possesses a physical and/or chemical attraction to the printing ink.
The coating formulation material comprises at least two portions: the insoluble pigment portion and the liquid binder portion. The pigment portion is typically kaolin clay in a liquid slurry. Other pigment materials, which may be used separately or admixed with kaolin clay, include titanium dioxide, calcium carbonate, barium sulfate, talc, zinc sulfate, aluminum sulfate and calcium oxide reaction products. The materials selected for the pigment portion impart a smooth white surface to the paper and modify brightness and ink receptivity of the coated paper. Generally the pigment slurry is milled under high shear until a very fine dispersion of the solids in the liquid phase is prepared.
To the pigment portion is added the liquid binder portion to improve the adhesive quality of the pigment to the paper. Generally, the binder comprises starches and protein material such as casein or soya protein and synthetic latexes. Also used as a binder is polyvinyl alcohol.
In preparing the coating formulation material, the binder, in aqueous or nonaqueous solution, is admixed with the pigment slurry. This binder-pigment mixture is then coated onto paper. A paper coating composition having a high-solids content is desirable because it reduces the amount of diluent present and consequently reduces the time and energy required to remove the diluent in finishing the paper. A high-solids paper coating composition would also improve coating effectiveness. Those in the paper coating field have long sought means to increase the solids content of a paper coating composition without deleteriously effecting the machining properties of the composition.